1. Field of the Invention
This invention relates to ink jet printers, and to a method of operating ink jet printers.
2. Description of the Prior Art
Ink jet printers are well known in the art. A typical such printer 10 is shown in FIG. 1. Hewlett Packard manufactures the Hewlett Packard thermal Ink-jet print cartridge 12, which may be used as the printing component of the printer 10. The Hewlett Packard thermal Ink-jet cartridge 12, well known in the art, is a self-contained disposable printhead suitable for a wide variety of dot matrix printing and plotting on a medium such as paper 13. The cartridge 12 includes a pressure contact electrical interconnect for ease of replacement, and can be used in any orientation.
Ink capacity of over ten million dots (dots are the ink drops ejected by the cartridge so as to print on the medium) gives the twelve nozzles 14 etc. of the thermal ink-jet cartridge 12 the capacity of typically 500,000 characters (depending on the font) or extensive graphics. The thermal ink-jet cartridge prints on a variety of papers (i.e., media) The non-contact operation allows variation in distance to the media when printing.
As is well known in the art, however, such ink jet cartridges as described above are subject to the problem of viscous plug formation. A viscous plug is a plug of ink in the exit portion of a print cartridge nozzle 14 in which evaporation of the liquid ink in the cartridge causes a thickening of the ink near the nozzle exit, interfering with ink dot ejection. This plug can occur when the print cartridge is idle (i.e., not firing) for a period of time, normally several days, depending on the ambient temperature and humidity conditions. The viscous plug can often be removed by printing. Plug formation is worst in hot and dry conditions but the plugs are most difficult to clear in cold and dry conditions. Plugs cause poorly formed dots or no dots at all and so degrade print quality.
The prior art (the Thermal Ink-Jet Print Cartridge Designer's Guide, Hewlett Packard, October, 1986) suggests that plugs which cannot be removed by firing (i.e., printing) will usually be cleared by holding a damp tissue in contact with the nozzles for several seconds and then gently wiping.
The same Designer's Guide also suggests that (page 10)
To insure immediate printing after an idle period, it is suggested that the following prefiring algorithm be used: PA1 Severe environments or specific applications may require more extreme measures. Some possibilities to try include: PA1 1. Increase the number of prefires. PA1 2. Decrease the maximum idle time. PA1 3. Increase the number of prefires at low temperatures. PA1 4. Insure that prefiring is done at maximum specified frequency. PA1 5. Control the temperature and humidity immediately around the print cartridge. PA1 6. Use a dampened tissue to "soak" and then wipe the nozzles when prefiring fails to clear a plug. PA1 7. "Cap" the print cartridge when not in use to maintain a closed humid environment around the nozzles. PA1 8. Place the cartridge in a sealed container when not in use. PA1 Periodic priming is not recommended for preventative maintenance. Only prime after a failure (see Maintenance Procedures).
After not printing (idle), and just before printing, fire all 12 nozzles several times; 1 firing for every 3 minutes of idle time up to a maximum of 160 firings.
In the prior art, the printer 10 includes a blotter 16 located to the left of the left-most printing location 18. The printhead guide track 20 contains a ramp (not shown) to lift the cartridge 12 over the blotter 16. Upon system initialization, the cartridge 12 is moved to the blotter 16 and fires all nozzles 14 for 160 times in succession, to perform the prefiring algorithm.
The Designer's Guide also suggests during high temperature periods, replacing the cartridge 12 every one to two months.
It is also suggested, for use outside the specification limits, to provide a tray of water near the head to raise humidity, and removing the print cartridge when not in use and storing in a cool location. The Designer's Guide specifically says that (page 10):
A common application of ink jet printers is as a printer system for use with a personal computer such as is indicated at 20 which provides data to printer 10 on channel 21. Under these circumstances, the printer 10 is usually turned off when not in use for long periods of time, thus making any sort of periodic priming impossible. Automated periodic priming under power on conditions even where possible has the drawback in most personal computer applications that it might disconcert the operator, due to unanticipated movement of the print head during the priming operation.